[FAQ] AM625: Crystal drive strength specifications

Hello Munan,

Thank you for the query.

We do not recommend connecting any series or parallel resistor on the SoC Xi and the Xo inputs. The connections are shown in the below section. 

Figure 7-16. MCU_OSC0 Crystal Implementation

The Cystal load capacitance and the recommended load cap recommendation are provided in the below section.

Table 7-21. MCU_OSC0 Crystal Circuit Requirements

The switching characteristics of the oscillator are provided in the below section.
Table 7-22. MCU_OSC0 Switching Characteristics - Crystal Mode

Customer will have to use these specification and work with th e crystal oscillator for choosing the right crystal.

Let me check if we have any additional information that could be shared.

Regards,

Sreenivasa

Hello Munan,

Thank you for checking.

I have sent a follow-up query to the expert to get his thoughts.

Regards,

Sreenivasa

Hi Sreenivasa,

Any update here? Customer still needs this information and is asking for us to escalate.

Munan

Hello Munan,

Thank you for following.

Please refer below:

The crystal power dissipation depends on crystal circuit variables that need to be considered.  The oscillators implemented in AM62x have an Automatic Gain Control (AGC) circuit that adjusts its drive strength to achieve resonance, which is another variable associated with crystal power dissipation.  Therefore, it is not possible for TI to define a power dissipation parameter for every crystal circuit implementation.  The customer would need to measure the voltage across their specific crystal circuit implementation and use the following formula to calculate power dissipated in the crystal.

 The crystal power dissipation can be calculated by the formula:

 P_xtal = 0.5 ESR (2 π F_xtal C_L V_xtal)², where ESR is the Effective Series Resistance of the crystal, F_xtal is the resonate frequency of the crystal, C_L is the capacitive load of the crystal circuit, and V_xtal is the peak-to-peak voltage across the crystal.

Regards,

Sreenivasa

Hi Sreenivasa,

Thanks for the additional detail here. But is there a max value? I assume that at some point we would be voltage or current limited out of these pins which would define a peak power spec that can be expected?

Munan

Hello Munan,

Thank you.

Customer will need follow the below section while choosing the crystal. 

7.11.4.1.1 MCU_OSC0 Internal Oscillator Clock Source

As the device expert mentioned above, we do not have a power dissipation value defined and that needs to be calculated.

Regards,

Sreenivasa

Hello Munan,

Based on your follow-up for additional info please refer the inputs i received from the device expert.

The crystal power dissipation will depend on the crystal they select and the crystal circuit (including the load capacitance) they implement. Our oscillator has an AGC that monitors oscillation of the crystal circuit and adjusts its gain to maintain oscillation. Therefore, it doesn’t have fixed drive strength.
Even if we told them our max drive strength, the power dissipation in the crystal would still depend on the crystal they select and the crystal circuit (including the load capacitance) they implement.
The formula for calculating crystal power dissipation in the crystal: Pxtal = 0.5 ESR (2 π Fxtal CL Vxtal).
Our oscillator could drive a voltage that approaches the potential of oscillator power supply. This is not very likely to occur with most crystal implementations. As you can see in the formula, the crystal power dissipation is still dependent on the ESR of the crystal circuit even if you assume a worst case value of 1.8V for Vxtal. Using a worst case value of 1.8V for Vxtal is very unrealistic for a typical crystal implementation since Vxtal is more likely to be 1.0V than 1.8V.

Regards,

Sreenivasa